TEMPE, Ariz. – A team of researchers from Arizona State University have found that warming resulting from megapolitan expansion is seasonally dependent, with greatest warming occurring during summer and least during winter. Among the most practical ways to combat urbanization-induced warming – the painting of building’s roofs white – was found to disrupt regional hydroclimate, highlighting the need for evaluation of tradeoffs associated with combating urban heat islands (UHI).

“We found that raising the reflectivity of buildings by painting their roofs white is an effective way of reducing higher average temperatures caused by urban expansion,” said Matei Georgescu, an assistant professor in ASU’s School of Geographical Sciences and Urban Planning. “However, increased reflectivity also modifies hydroclimatic processes and, in the case of the ‘Sun Corridor,’ can lead to a significant reduction of rainfall.

The research is presented in the paper, “Seasonal hydroclimatic impacts of Sun Corridor expansion,” published in the Sept. 7, 2012 issue of Environmental Research Letters. Georgescu, the lead author of the paper, is joined by Alex Mahalov, The Wilhoit Foundation Dean’s Distinguished Professor in the School of Mathematical and Statistical Sciences at ASU, and Mohamed Moustaoui, an associate professor in ASU’s School of Mathematical and Statistical Sciences.

The paper focuses on Arizona’s Sun Corridor, the most rapidly growing megapolitan area in the United States. Located in a semi-arid environment, the Sun Corridor is composed of four metropolitan areas: Phoenix, Tucson, Prescott and Nogales. With a population projection expected to exceed 9 million people by 2040, the rapidly expanding megapolitan offers the opportunity to identify tradeoffs focused on sustainable expansion of the built environment.

The authors utilized 2050 projections of Sun Corridor growth developed by the Maricopa Association of Governments (MAG), the regional agency for metropolitan Phoenix that provides long-range and sustainably oriented planning. They conducted continuous multi-year, multi-member, continental scale numerical experiments for several 2050 Sun Corridor growth and adaptation scenarios and compared results with a modern day Sun Corridor representation.

“For a maximum expansion scenario, we find greatest warming to occur during summer, in excess of 1 degree C (1.8 degrees F) when averaged over the entire state of Arizona. Warming remains considerable during both spring and fall seasons, approaching 0.9 C. For a minimum expansion scenario, the consistent theme of maximum warming during summer with reduced, although still significant, warming during spring and fall seasons persists,” Georgescu added.

Whereas previous research has documented the contribution of cool roofs as an effective UHI mitigation approach, this work emphasizes the need to broadly evaluate impacts by exploring consequences that extend to hydrology and rainfall.

“Truly sustainable development will have to consider impacts extending beyond average temperature,” Georgescu explained. “A crucial step in that approach is to identify potential adaptation and mitigation strategies and assess tradeoffs, to ensure that we make smart decisions with minimum damaging consequences.”

###

All three co-authors are affiliated with ASU’s College of Liberal Arts and Sciences and the Global Institute of Sustainability.

This study looks pretty weak to me. I couldn’t understand very well the methodology, but I don’t see how they can reach that conclusion. Was it through experimental data or through modelling? In the first case, correlation is not causation. In the second, well, we have learnt already a number of things about climate models reliability, haven’t we.

Cover the roofs in heat collectors. You can incorporate them into the backs of solar photovoltaic panels.

Store the summer heat deep underground, use it for heating when needed around winter and for heating water year round.

Makes more sense than fighting the warmth by pumping CO₂ underground.

What’s the worse that could happen? In warmer sunnier places like the Sun Corridor, so much heat will get pumped underground that the deep stores will generate steam, that could drive a turbine and generate electricity?

OK, I have not read the research in question, but this figure seems out of line to me. Taking any significant geographical micro-climate type area (as in 500 square miles plus) even in an “urban” area in Arizona (which isn’t exactly = to the five boroughs of NYC) it’s hard to imagine that total roof area would be more than 25% or so to begin with at most. A lot of those roofs are already more gray than black (with a certain percentage already white I’m guessing as well), and the painted white wouldn’t stay nearly 100% reflective for very long, so I’m purely guessing that we’re talking about a reflectivity change of about 50% for that less-than-25% area — or a reflectivity change of 12.5% for the total area. Is rain production really 1/3 dependent purely upon reflectivity input into the microclimate (which is what would be needed for that “4%” final result)? Heh, and of course there’s the energy angle: the reduced rain would lead to increased air-conditioning, while the white roofs would lead to decreased air-conditioning! And white roofs might be more (or less) energy-expensive to produce/maintain.

If we were talking about the entire state of Arizona being urbanized to NYC level standards it would be something else, but given the reality of the situation I have a hard time believing any effect would be significant enough to really worry about. It *is* an interesting concept though! And if other costs were commensurate it would certainly be worth considering this sort of calculation if they were deciding about blacktop vs. whitetop road material etc.

It is a rather good idea on paper at least to combat UHI. What’s needed though is a surface that can change light reflectivity but remain cheap. For example a photoreactive paint, where it gets brighter with more light would be preferable, but I’m not sure if such a thing would have to be invented first.
An easier, but perhaps a more expensive solution would be to install a Venitian blinds-style electronic roof covers, that turns itself on when it’s needed.

P.S. If all the tarmac in a urban environment was coloured green for example, would that have an effect on the local temperature too? Would it have the same consequences as the white roof?

California naturally showed everyone The Way when director of general services in 2009 ordered “white as the default color on the state’s light-duty vehicle contracts”. Science now unsettling with unintended consequences worth calculating, since Ca. total state fleet is over 50,000 ( not all are light-duty vehicles). Seems impossible though to calculate the total surface area of Ca. commercial buildings (since 2006) with a flat roof being obliged to be white or light (reflective) shade.

But I can assure you this would be marketed as fair and equitable, as the costs would be borne by taxes on the hated “black burners” who insist on destroying the biosphere by continuing to use fossil fuel-derived energy.

And if that term sounds “racial”, don’t worry because they won’t worry about it as they gladly promote the implication that anyone who isn’t willing to do anything possible to combat climate change must obviously be of that political persuasion who’d want to do that as well. “Drill Alaskan oil, Kill African baby” and similar protestor-sign slogans will, of course, be expected.

I’ve done Phoenix and Tucson in the summer. I don’t see another one degree making a whole lot of difference. And there is one awful lot of desert and mountains in that ‘corridor’ which will overwhelm any effort of roof-painters.

I vaguely recall that green (dark) was nearly as good as black for solar water heater panels. I had an old Chevy that was forest green, and it got too hot to touch sitting in the sun. Of course, correlation doesn’t mean causation.

Long ago the UK’s Coal Research Establishment (CRE) developed a range of roofing materials. One of these had an aluminium coating (like baking foil) for thermal reflectivity and low thermal emissivity

As anticipated, the reflectance of the coating rapidly reduced due to oxidation (the silver colour tuned to grey), but the remaining reflectance was (as had been calculated) a sufficient benefit over a tarred roof for the coating to have merit.

However, the reflectance continued to degrade as a result of dirt, bird droppings, algae, etc. accumulating on the surface. It was clear that similar accumulation would occur on tarred roofs. Hence, within about a year following installation there would be surprisingly little difference between the reflectance of a flat roof covered with a ‘traditional’ tarred roofing felt and and a flat roof with the aluminium-coated roofing material. And the ‘traditional’ roofing felt was cheaper.

So, on the basis of existing experimental data (which I helped to acquire) I think the effect of painting the roofs would be much less than the reported model study suggests.

Also, the anticipated reduction to rainfall is only indicated by use of a model. Were anybody willing to provide me with sufficient funds then I would be willing to conduct a model study which indicates the painted roofs would increase rainfall (i.e. give me the money, tell me what you want, and I can give you a model which provides what you want).

If white roofs tend to lower the temp toward its “natural” level, and that reduces rainfall, does that mean that urbanization has actually increased rainfall??? Plausible, I suppose, since urban areas use more water, and a lot of it evaporates, and evaporation presumably increases with temperature.

“The authors utilized 2050 projections of Sun Corridor growth developed by the Maricopa Association of Governments (MAG), the regional agency for metropolitan Phoenix that provides long-range and sustainably oriented planning. They conducted continuous multi-year, multi-member, continental scale numerical experiments for several 2050 Sun Corridor growth and adaptation scenarios and compared results with a modern day Sun Corridor representation.”

Liberal use of the word experiment.
Let me rephrase that:

They conducted continuous multi-year, massively multiplayer, continental scale role playing games for several 2050 Sun Corridor growth and adaptation levels. After they ran out of doritos, lead researcher Matei Georgescu, an assistant professor in ASU’s School of Geographical Sciences and Urban Planning, uploaded the recorded gameplay to youtube, declaring a record.

“While it is true that cool roofs are mostly associated with white roofs, they come in a variety of colors and materials and are available for both commercial and residential buildings. Note that today’s “cool roof” pigments allow metal roofing products to be EnergyStar rated in dark colors, even black. They aren’t as reflective as whites or light colors, but can still save energy over other paints.

“A 2011 study by researchers at Stanford University suggested that although reflective roofs decrease temperatures in buildings and mitigate the “urban heat island effect,” they may actually increase global temperature. The study noted that it did not account for the reduction in greenhouse gas emissions that results from building energy conservation (annual cooling energy savings less annual heating energy penalty) associated with cool roofs.

” A response paper titled “Cool Roofs and Global Cooling,” by researchers in the Heat Island Group at Lawrence Berkeley National Laboratory, raised additional concerns about the validity of these findings, citing the uncertainty acknowledged by the authors, statistically insignificant numerical results, and insufficient granularity in analysis of local contributions to global feedbacks.”

“Trees and vegetation lower surface and air temperatures by providing shade and through evapotranspiration. Shaded surfaces, for example, may be 20–45°F (11–25°C) cooler than the peak temperatures of unshaded materials.1 Evapotranspiration, alone or in combination with shading, can help reduce peak summer temperatures by 2–9°F (1–5°C).2, 3

Trees and vegetation are most useful as a mitigation strategy when planted in strategic locations around buildings or to shade pavement in parking lots and on streets. Researchers have found that planting deciduous trees or vines to the west is typically most effective for cooling a building, especially if they shade windows and part of the building’s roof.”

The easiest way to compensate for the lack of rain induced locally by these white rooftops is to hold dances on the rooftops daily. The researchers from Arizona State University should have access to a talent pool capable of teaching the student to perform the traditional rain dance.
/sarc

Steel roofs are a better option as they reflect solar energy outside the visible spectrum as well as the visible spectrum. Take a look at where I live, south of the river, Perth, on Google Earth. More than 2 out of 3 roofs look white, but in fact they are steel.

As for ‘Sun Corridor expansion scenario leads to a 12% reduction in rainfall’, I don’t know what assumptions were made to reach this conclusion, but Roy Spencer found in the USA southwest urban temperatures decreased with urban density. He suggests and I agree, that the cause is decreased vegetation (and irrigation) and hence reduced humidity.

So the 2 rainfall reductions are due to different causes. One due to fewer plants (and irrigation0, hence reduced humidity. The other due to reduced temperatures (from white roofs) and reduced thermal convection.

According to weather.com the average annual rainfall for Tempe is 9.37 inches. A 4% reduction would be .37 of an inch. Red flags should go up when percentage is used instead of actual numbers. As they say, figures lie and liars figure.

The other unintended consequence … thanks to the law of consumer gravity, everything, especially including mass quantity suppliers inevitably run a race to the bottom. Billions of gallons of white paint will necessarily be supplied by the lowest bidder using the cheapest labor and the loosest standards.

So just imagine white roofs all over the world leeching particles of white paint made in China into the local ground water tables. Yeah, that’s a great idea. Eco-Nazis or Eco-Frauds? We report, you decide.

Looks like I was a little too hasty in my comment (early in the AM). Noting that the area in question is not Tempe but rather Phoenix/Tuscon/Prescott/Nogales. However using the link just change the city and you can see the precipitation for all those cities. The same basis of my post still holds, 4% of very little is very little.

Sod roof coverings are a cultural tradition here, and now at the crux of a business conflict. The grasses are cropped by goats but their use has been ‘trademarked’ and successfully protected in local courts.

Meanwhile, locally, phragmities are burned and poisoned even though reed is the traditional thatching of choice. Phragmities thatched roofs last hundreds of years with little maintenance, and have a significant insulating ‘R’ value, and sequester carbon at almost tonne for tonne.

What about all that white stuff that covers roofs and everyhing else during the N.H. winter? Whole regions of the U.S., N. Europe, Canada, etc. are white and it doesn’t seem to affect precipitation much one way or the other.

Anyway, how could you tell the difference in Phoenix AZ where you don’t get squat for rain? “We had 30% less than none?”

In a real-world 2008 case study [10] of large-scale cooling from increased reflectivity, it was found that the Province of Almeria, Southern Spain, has cooled 1.6ºC over a period of 20 years compared to surrounding regions, as a result of polythene-covered greenhouses being installed over a vast area that was previously open desert. In the summer the farmers whitewash these roofs to cool their plants down. http://en.wikipedia.org/wiki/Cool_roof

Perhaps urbanization, done properly, would be good for the environment.

One “geoengineering” proposal for reducing the impact of urban heat islands is to paint roofs worldwide a reflective white. Jacobson’s computer modeling concluded that white roofs did indeed cool urban surfaces. However, they caused a net global warming, largely because they reduced cloudiness slightly by increasing the stability of the air, thereby reducing the vertical transport of moisture and energy to clouds.http://www.eurekalert.org/pub_releases/2011-10/su-ui101811.php

I haven’t read a good description of how urban heat islands affect the “stability of the air” and how exactly this affects “cloudiness” in any scientific paper. But hang gliding enthusiasts are aware of the phenomenon and explain it better.

In a nut shut, thermals are rising columns or pockets of air that are caused by uneven heating of the ground surface. As you know, different color object absorb more heat from the sun than others. One a hot sunny day you’ll notice the surface temperature off asphalt is much higher than the grassy area next to it.

Naturally occurring thermals will most likely form over sandy or rocky surfaces, freshly plowed fields, or any place where the surface is likely to be hotter than its surroundings.
[…]
You will find the strongest thermals when the sun is at its highest on nice sunny day. The strength of thermals is directly proportional to the variation of surface temperature. This means the strongest thermals are most likely to occur on a warm sunny day after a relatively cool night.

The “heat islands” that are clearly recognisable in satellite pictures impact convection and so have an effect on the wind system in the surrounding area. … However, conurbations have a special role not only where causes are concerned but also when it comes to the impact of the weather factors of wind, precipitation and temperature. Here, torrential rainfall can soon lead to local flooding, and even to devastating flash floods and landslides, which then affect mostly the poorer social strata.http://web.archive.org/web/20080131223004/http://www.munichre.com/publications/302-04271_en.pdf

It appears that neither the Stanford or Arizona team draw much attention to the fact that, according to their own assertions, the clouds and rainfall resulting from urban heat islands are anthropogenic. And that white roofs would mitigate this anthropogenic effect. I thought that was supposed to be a good thing in the worldview of the new eco-religion. I suspect their computer models (and/or press releases) need a bit more tweaking.

The predictions were purely based on modeling. From the abstract: “Based on a suite of ensemble-based, multi-year simulations using the Weather Research and Forecasting (WRF) model, we quantify seasonally varying hydroclimatic impacts…”.

On the positive side the paper implicitly acknowledges that the climate impact of changing land use are predicted to be as large or larger than claimed GHG impacts. Also, the paper is based on a weather forecasting model (WRF [ARW] version 3.2.1) rather the CGMs used for most other studies. This model has demonstrated skill in forecasting near term (e.g., hurricane tracks) and seasonal weather parameters. I don’t have much confidence in the three year predictions used in the study. However, it is plausible that the model’s parametric response (e.g., temperature, precipitation) to land use changes (e.g., permeability, albedo) can tell us something.

In addition, the principle that we should assess likely second (and third) order effects of potential/proposed solutions (to any problem, not just climate related issues) is IMO very sound.

On the negative side, the methodology is very weak and basically boils down to “we ran the model a few times and reported the results without much explanation or thought. The number of replications used in each case is not clearly stated (as few as 12?) and no variance is stated for temperature or precipitation values. Figure 5 does provide error bars for evapotranspiration. By visual comparison, the difference between the adaptive (white roof case) and high case is less than the 1 standard deviation bound for each case indicating that the effect is not statistically significant. No error bars are provided for precipitation which is the parameter we presumably care about. From Figure 7, the three year precipitation difference between the adaptive and high cases appears to be about 70mm or 5% of the toal for the control case. Again, likely not statistically significant.

No data is provided for the assumed roof albedo for the non-adaptive case, the fraction of urban surface area that is assumed to be roof, or the net change in regional albedo between the four cases.

There is no attempt to reconcile the statement in section 3.1 that “For all seasons (except winter, when cooling is simulated) urban-induced warming remains an entirely local phenomenon, and non-urbanizing locales undergo little or no warming.” with the statement in section 3.2 of “… potentially significant consequences via modification of the regional hydrologic cycle associated with this adaptation approach”.
The paper also fails to offer a physical explanation of why use of white roofs would decrease evapotranspiration and precipitation for a given surface permeability.

It is also disappointing that the authors felt the need to include a reference usual GHG induced AGW canard with the statement that “In addition, these impacts are layered on top of large-scale climate change owing to increased emissions of greenhouse gases, adding further stress to an already sensitive system.” in a paper that has nothing to do with this topic. I suppose that this obeisance is a prerequisite for publication in a climate .

The energy saving potential of white roofs has been over stated. Installing a coating on a poorly insulated roof will make a difference (over 5% in one project I worked on). However, if the roof is very well insulated, the reflective coating may only give you an additional 1% improvement, which won’t give you a good return on investment for the material cost and labor.

Now if you had HVAC equipment on the roof it would reduce the load on it. How much? No idea.. I have never come across a measurement and verification study looking at just that.

The biggest advantage of a white roof.. it makes the roof good for a deck and plants as it reduces the surface temperature, even in blinding sun, to tolerable levels. My own roof garden did very, very well this year (though it got too hot for some plants). The CO2 levels typically hover around the 500 to 650 ppm range, which does give you a definite boost in growth.

I see others beat me to the obvious observation of “unintended consequences”… because it’s a special kind of fun to point out where someone who thinks they’re “saving the planet” end up messing things up for real (that are not really messed up to begin with).

Now, if this is the result of something as relatively innocuous as white roofs, just image the possible unintended consequences of the more esoteric or ridiculous mega projects like:
* SO2 scattered into the upper atmosphere
* pumping gigatons of Co2 into underground caverns
* dumping iron filings into the ocean
* giant orbital mirrors
* giant wind farms, giant solar installations,
* etc….

Honestly, this is the best part of being old enough to remember the Energy Crisis and Global Cooling scares of the 70s… watching people make the same stupid decisions, wasting money on the same schemes, trying the whole thing all over, and expecting different results.

Of course, the major downside is the poorly informed governments that are handing out OUR money to these wasteful attempts to change something that is not in our power to change…

@richardscourtney
“However, the reflectance continued to degrade as a result of dirt, bird droppings, algae, etc. accumulating on the surface. It was clear that similar accumulation would occur on tarred roofs. Hence, within about a year following installation there would be surprisingly little difference between the reflectance of a flat roof covered with a ‘traditional’ tarred roofing felt and and a flat roof with the aluminium-coated roofing material. And the ‘traditional’ roofing felt was cheaper.

So, on the basis of existing experimental data (which I helped to acquire) I think the effect of painting the roofs would be much less than the reported model study suggests.”

NOT if a bylaw is passed requiring all owners to maintain a minimum of 60% reflective efficiency, on penalty of a $5000 fine. And for those who can’t climb up to their roofs for medical reasons, a $200 roof tax will be applied to pay for support staff.

mwhite says:
September 8, 2012 at 3:37 am
—————————————-
On my way into town I drive through a large well-treed marshland. The outdoor temperature (as shown by my car) drops at least a couple of degrees. By the time I get to the city, especially when I park in an asphalt covered parking lot, the temperature can be 4-6 degrees warmer than at my house. When we build cities we cut down all the trees, fill in the marshlands and then pave them over with asphalt. I don’t want to sound like Joni Mitchell but we should stop worrying about global warming and start worrying about city warming. We are cultivating our cities to be 3-5 degrees warmer than they should be.

Hot-arid is not my area of experience, but their reference to “impacts on the hydrologic cycle are aggravated via enhanced evapotranspiration reduction” seems a bit odd. If the area they are talking about is hot-arid, then the dominant flora will be resistant to moisture loss due to transpiration. If this is replaced by buildings, with people in them breathing, perspiring, cooking, etc then the resulting moisture will either be extracted to the air outside or recovered by the building systems.

3 thoughts come to mind immediately :
1) This is a model, not hard data. We all know the limits of climate models & how much faith we should put in them (not much).
2) Heat – If the desert urban area is growing so much, evidently these are heat-loving people. Why are they even concerned about reducing the urban heat island?
3) Adaptation – If that many people can adapt to summer time desert heat, why is anyone worried about a degree or two of AGW – we’ll adapt just fine :))

two things:
can you imagine driving the run from los angeles to phoenix in september/october with a bright white road. the eyestrain and headaches would be houmoungas.

these twits that advocate light colored roofs (anything other than light grey is not practical over even the short term) have never looked out an aircraft on approach to LAX. there is a good 40-50 miles of industrial/commercial buildings with white/light roofs and they have been that way for decades.

go to the local hardware store. look at the “rolled roofing”. as i remember in the hot parts of the country there are only two colors offored. light grey and lighter grey.

these guys are floogging a horse thats been dead and gone for nearly a century.

As has been pointed out, the authors didn’t bother to establish the statistical significance between “white” roof tops and rainfall. They just assumed that there was a statistically significant correlation and went from there. It is not the mathematicians fault for not noticing this since he is not a scientist, he just got asked to work the math associated with the problem. However, this is a typical representation of the Liberal Arts degrees in Climate Science and why I have a major issue with the University that I am an Instructor for. No science degree should ever be offered as an Arts degree as is all of the new “green” politically motivated science degrees are.

Is this article about the climate inside these buildings?Or,is it about the climate downwind of their UHI’s?Does the air move in there models?…1 degree?…WUWT?
Thanks for all the interesting articles and comments

First, we need to ascertain just how stable temperature is supposed to be. Then we need to know what raising it in limited areas does. Iowans were delighted when the wind turbines warmed crop lands as they protected against freezing for short time into the fall. So in this case, we are happy to make things warmer….
Second, if we paint roofing white in New Hampshire, we may avoid urban heat sinks, but we raise the cost of heating during the 9 or so months of winter. So we keep the city cool, but use up more energy to do so. This entire piece does indeed point out how unintended consequences arise when people look at only one or two variables in a situation that has hundreds, then declare that their variable is what needs addressed.

To me quite hard to believe it could have any such a significant effect … and it is even harder for me to believe they could have any certainty at all about their ability to calculate the magnitude of this effect with any degree of accuracy.

And it also seems to imply that surface warming (via UHI) would result in more rainfall – usually a plus in most situations.

“Trees and vegetation lower surface and air temperatures by providing shade and through evapotranspiration. …”

The solution: “Just add water”

Not always an option (speaking as one who has lost trees these last two summers in Tejas) due to scarcity of water (we were on a once-a-week watering regimen in DFW area last year owing to drought and other factors).

The energy saving potential of white roofs has been over stated. Installing a coating on a poorly insulated roof will make a difference (over 5% in one project I worked on). However, if the roof is very well insulated, the reflective coating may only give you an additional 1% improvement, which won’t give you a good return on investment for the material cost and labor.
…

Are you talking savings in HVAC costs ?

The article would seem to be addressing albedo (diffusely ‘reflecting’ incoming solar energy; but I cannot vouch for that ATTM.)

Steve from Rockwood says September 8, 2012 at 6:09 am
…
—————————————-
On my way into town I drive through a large well-treed marshland. The outdoor temperature (as shown by my car) drops at least a couple of degrees. By the time I get to the city, especially when I park in an asphalt covered parking lot, the temperature can be 4-6 degrees warmer than at my house. When we build cities we cut down all the trees, fill in the marshlands and then pave them over with asphalt. I don’t want to sound like Joni Mitchell but we should stop worrying about global warming and start worrying about city warming. We are cultivating our cities to be 3-5 degrees warmer than they should be.

Amen. Similar/same effect noted here when driving in from the outskirts of the city (suburbia) back into denser civilization, and the change is from prairie (rather than marshland) to ‘city’ …

Maybe you can help me here, but, my experience is that steel roofs (*) run physically quite hot; is there some sort of surface treatment that is required?
.
.
(* Contrast this with say a lush, green lawn which actually feels cool when one’s hand is placed upon it during mid-day even.)

I’m not buying this unless the aggregate albedo is raised above the natural background albedo. Assume no buildings there. There is some natural albedo. Now you build a lot of black parking lots, roofs and highways and the aggregate albedo of the area drops. Now you start painting some roofs white. Until you get the aggregate albedo of the city above what it was naturally, which I suspect would require painting quite a lot of roofs to compensate for the unpainted roofs, streets, and parking lots, you aren’t going to have a negative impact on convection (which is probably the rainfall impact).

However, the reflectance continued to degrade as a result of dirt, bird droppings, algae, etc. accumulating on the surface. It was clear that similar accumulation would occur on tarred roofs. Hence, within about a year following installation there would be surprisingly little difference between the reflectance of a flat roof covered with a ‘traditional’ tarred roofing felt and and a flat roof with the aluminium-coated roofing material. And the ‘traditional’ roofing felt was cheaper.

No. A model is a hypothesis. An experiment would be what you would do to test your model. Your “numerical experiments” tell you nothing about the world. They only tell you how the model you created behaves.

Not necessarily, a good conclusion in my view. What if you had a black roof, with good foam thermal insulation under it. So the blacktop can get nice and hot without frying the building, and then it can radiate much better at that higher T^5 Temperature shifted peak, and also heat the air by conduction, which leads to convective cooling.
Well lotsa ways to skin a cat.

And to go along with Louise; UHIs are good for cooling the earth faster; but you have to get out of that Hansen habit of measuring the TEMPErature, of an Arizona Parking lot thermometer, and using it as the TEMPErature of some place 1200 km away from there, and claiming thw whole area is hot.

When I lived in Puerto Rico (1968-1971) we white washed our flat roofs every dry season, i.e., the winter. No problem with rainfall there. Ever.

People would not spend the time, money and effort painting the roofs each year if that were not necessary. It gives real-world confirmation to our experimental findings in the 1980s which I report in my above post at September 8, 2012 at 2:19 am. Clearly, we could have saved much effort if we had known what you and your neighbours knew.

And it seems the authors of the above model exercise reported in the above article still don’t know it.

Maybe you can help me here, but, my experience is that steel roofs (*) run physically quite hot; is there some sort of surface treatment that is required?

Whether the roof gets hot or not isn’t really relevant. Its the energy flows that matter. The most important in hot areas is reflectance (albedo). The second factor is emissivity. These 2 factors determine how much heat energy gets past the steel roof and hence into the house.

The person who invents a roofing material that changes from low to high albedo (reflectance) when temperatures warm past about 25C is going to make a fortune.

And, the reason vegetated areas and swamps are cooler is higher humidity, even though because of lower albedo they have more heat energy than urban surfaces. Which is why temperature is a meaningless metric when comparing the heating of urban versus non-urban areas.

Philip Bradley says September 8, 2012 at 4:05 am
“Steel roofs are a better option as they reflect solar energy outside the visible spectrum as well as the visible spectrum. Take a look at where I live, south of the river, Perth, on Google Earth. More than 2 out of 3 roofs look white, but in fact they are steel.”

Phil,

We have a brown colored steel garage door (facing east) at our place in the Sierra foothills. The combination of the brown color (which matches the cedar siding color on the building) with the rather poor insulation characteristics of the metal door led to a huge heat transfer into our garage until about 11:45 am every morning in the late spring to early fall. We found that using a UV shade (installed with about a 6″ air gap between the garage door and the shade) prevents the suns mix of wavelengths getting to the metal door which has made it possible to do some temperature sensitive activities in the garage.

What exactly happenes to to the energy in the different wavelengths of light from the sun I am not to sure about. The temperature of the shade doesn’t seem that hot when I touch it. I assume some of the wavelengths get reflected away from the shade at a many different angels. Our gravel parking/driving area doesn’t seem any hotter in front of the garage, but I have never actually measured the temperature. In any case the shade does it’s job in the spring to fall time frames and we roll it up to let the suns rays warm the garage in the winter.

Seems like a well-controlled experiment should be possible. Find one of those semi-abandoned military bases fairly close to a city. Keep good temp and precip records there for a year, and calculate deltas from the city’s readings.

Paint all the roofs on the base white, or at least all the big flat roofs. Keep the same records the next year. See if the base is now cooler or wetter than before, as delta from the city’s values.

Philip Bradley: “The person who invents a roofing material that changes from low to high albedo (reflectance) when temperatures warm past about 25C is going to make a fortune.”

It’s called a parasol, isn’t it? Though to stay with the zeitgeist one would need thermistors, Arduino boards, rare-earth magnets, and a research grant from DARPA. To keep seasonal issues in check one needs only an RTC chip. And for wind related wear issues one needs only keep about a miniature turbine to detect wind-speed and provide electrical cogeneration needs for recharging keeping oil-cell magnesium/carbon battery arrays up. For wind intermittent issues one can use a Joule Thief charger driven by an earth battery.

Certainly less absurd than Solyndra and contains a higher buzzword-to-content ratio as well. Maybe enough to get an NGO to sport funds for inflicting this on Africans as a live human test.

Well such an experiment would reveal if / or not the base was cooler or wetter than before.

It would reveal nothing else, as it simply isn’t possible to rerun last year’s weather conditions over again this year after making alterations to the experimental apparatus; and that I’m afraid is the problem with the whole of climate “science”

In experimental science if you controllably and measurably alter an apparatus; and then submit it to an uncontrollably and unmeasurably different stimulus signal, you have the poster child for the “garbage in; garbage out” condition.

That sort of experimentation leads to conclusions similar to the one reached after cutting all four legs off a frog to see how that impairs their jumping ability; and their complete failure to jump “proves” that sans all four legs, frogs become stone deaf, and simply ignore your shouts of encouragement.

The main problem is that even with something as simple as painting roofs a different colour, no one can agree on whether t is good or bad.
Of course, apart from the paint companies, the big beats of AGW don’t make any money out of this so don’t expect them to rush to the rescue.
On the other hand, if the research is right and that the correlation can be found to have a causative link, then it just shows that if they can mess up painting roofs, think how much worse they can mess up with all their other strategies.
OK, what happens if, following a temperature rise they manage to suppress or partially suppress the usual consequent CO2 rise?
Maybe we will then find that the CO2 rise actually is a part of another mechanism that counter-intuitively has a role in recovery from warming.

Here in the mid-Atlantic, it prb’ly saves alittle money to have a dark roof since the heating season is much longer than the cooling season. But a white roof might extend the lifetime of an asphalt-shingle roof a bit — heat degrades materials.

Also, IMO it’s not necessarily the color of the roofing surface it’s the nature of irregular surfaces. In nature you have terrain, rocks, trees, leaves on the trees, etc. They to not absorb at the same rate nor to they reflect in the same uniform directions.

@EM Smith, the problem with planting on rooftops is that is requires much more in maintaining it. The whole idea of painting rooftops is to minimize costs and seal possible rain leakage. Putting thousands of pounds of water saturated dirt and plants is quite a risk.

“It’s called a parasol, isn’t it? Though to stay with the zeitgeist one would need thermistors, Arduino boards, rare-earth magnets, and a research grant from DARPA. To keep seasonal issues in check one needs only an RTC chip. And for wind related wear issues one needs only keep about a miniature turbine to detect wind-speed and provide electrical cogeneration needs for recharging keeping oil-cell magnesium/carbon battery arrays up. For wind intermittent issues one can use a Joule Thief charger driven by an earth battery.

Certainly less absurd than Solyndra and contains a higher buzzword-to-content ratio as well. “

The level of absurdity is always quantifiable by putting the complexity of the apparatus in relation to the energy density of the energy source. By that measure, even Solyndra beats your Rube Goldberg device. Solyndra was probably only a factor of three or so worse than ordinary thin film cells, which of course was sufficient to make them perish in the marketplace.

JJ says:
September 8, 2012 at 11:14 am
“Now, models are called “experiments”.

No. A model is a hypothesis. An experiment would be what you would do to test your model. Your “numerical experiments” tell you nothing about the world. They only tell you how the model you created behaves.”

Exactly, or nearly.
The model is the theory (it is the formula).
The model run is the prediction of the theory, or the hypothesis.
Reality is the experiment to test the hypothesis.

After the Ice Age for a period of about 4,000 years the Sahara had vegetation which increased heat absorption. The greater warmth increased the volume and rapidity of rising hot air, resulting in moist air being drawn in from the ocean and seas. This increased rainfall, which may be the same process as in Arizona. White roofs would reduce UHi, and thereby reduce rising hot air and the drawing in of moist air.